An automotive automatic transmission generally includes a transmission mechanism having planetary gears which include speed-changing elements (hereinafter referred to as gears) such as sun gears and planetary carriers, and hydraulic friction engaging elements such as hydraulic wet type multiple disk clutches and hydraulic band brakes (hereinafter referred to as clutches and brakes, respectively). In an automatic transmission of this type, a shift position connection is changed from a first speed to a second speed by releasing one(s) of the friction engaging elements associated with the first speed and by engaging another friction engaging element(s) associated with the second speed, to select gears which contribute to torque transmission, to thereby establish a desired shift position.
In recent years, in order to improve the drivability of automobiles and reduce the fuel consumption, attempts are made to further enhance the degree of electronic control for the automatic transmission and increase the number of shift positions or transmission stages. A typical multistage automatic transmission includes a main transmission mechanism having an existing transmission mechanism and a subsidiary transmission mechanism coupled in line, with respect to torque transmission, with the main transmission mechanism. Those two transmission mechanisms which contribute to torque transmission are combined in various manners, to thereby establish an arbitrary one of a required number of shift positions, e.g., five forward speeds and one reverse speed. For example, the speed position in the main transmission mechanism is changed over to effect a speed-changing operation among a first speed, a second speed, and a third speed, and the speed position in the subsidiary transmission mechanism is changed over to effect a speed-changing operation between the third speed and a fourth speed. Further, to carry out a speed-changing between the fourth speed and a fifth speed, the speed position in the main transmission mechanism is changed over, with the gear engagement state (torque transmission path) in the subsidiary transmission mechanism set to correspond to the fourth speed. In other words, by use of the subsidiary transmission mechanism, a switching is made between the lower-speed shift positions including the first through third speeds and the higher-speed shift positions including the fourth and fifth speeds.
In an electronic controlled automatic transmission, a shift map determined as a function of vehicle speed and a throttle valve opening degree, as shown in FIG. 8, is generally used to select a shift position. From this map, an optimum shift position (target shift position) suitable to detected values of the vehicle speed and throttle valve opening degree is selected. In the case of a kick-down at a rapid acceleration, the target shift position is generally determined by the throttle valve opening degree. That is, when the throttle valve opening degree traverses the 5-4 shift line or 4-3 shift line as shown in FIG. 8, a down-shift command is output. As a result, if the accelerator pedal is depressed by the driver and the throttle valve opening degree .theta. reaches the point B from the point A in FIG. 8, a down-shift is carried out from the fifth speed to the fourth speed. When the throttle valve opening degree .theta. reaches the point C from the point A, a so-called skip down-shift from the fifth speed to the third speed is effected.
On an occasion that a skip down-shift from the fifth speed to the third speed is effected in the aforementioned multi-stage automatic transmission, it is necessary to make the changeover of speed positions in both of the main and subsidiary transmission mechanisms. However, it is generally extremely difficult to simultaneously control a plurality of transmission mechanisms. For example, if both of the main and subsidiary transmission mechanisms are simultaneously controlled at the time of a skip down-shift, the release operation timing of the friction engaging element on the released side of the main and/or subsidiary transmission mechanism may be sometimes deviated from the optimum timing. In such a case, the turbine rotation speed of a torque converter of the automatic transmission (the rotation speed of the input shaft of the main transmission mechanism) rapidly rises. As a result, there is a fear that a large shift shock occurs or the concerned is broken when the friction engaging element on the engaging side of the main and/or subsidiary transmission mechanism is brought into engagement.
Further, in the automatic transmission, the speed change control is generally effected based on the outputs of two rotation speed sensors respectively indicating the rotation speeds of the input and output shafts of the transmission. However, the speed-changing condition in each of the main and subsidiary transmission mechanisms cannot be detected based on the outputs of the two sensors. Therefore, it is extremely difficult to simultaneously control the speed-changing operations in both of the transmission mechanisms based on the two sensor outputs. Provision of additional rotation speed sensors makes it possible to detect the speed-changing condition of each transmission mechanism, and, in turn, to simultaneously control a plurality of transmission mechanisms. In this case, however, the cost rises.
Conventionally, therefore, in order to carry out a skip down-shift, the speed position in the main transmission mechanism is first changed over, and then the changeover of the speed position is made in the subsidiary transmission mechanism. For example, for the down-shift from the fifth speed to the third speed, a method has been adopted in which the fourth speed is temporally established during the downshift, so that the downshift is carried out by way of the fourth speed. However, this type of speed change control method entails the following defects.
As is well known in the art, a hydraulic piston serving as driving means for a hydraulic friction engaging element has play or ineffective stroke. This requires a so-called play eliminating operation which must take place before the operation of releasing or engaging the clutch or brake is actually started. Therefore, even if a down-shift command for instructing a shifting to the third speed is issued immediately after temporarily establishing the fourth speed, the fourth speed is kept established during a time period in which the play eliminating operation is carried out. Further, to establish the fourth speed, it is necessary to make a time-consuming determination as to the synchronization of rotation speeds of the input and output shafts of the transmission. According to the conventional method in which a skip down-shift is made by effecting a one-step down-shift a plurality of times, a shift shock occurs each time the one-step down-shift is effected and a period of time required for shifting becomes longer. Therefore, the ride qualities and shift response are degraded.